s s6. () Caleulate the rate of heat conduction in watts out window is, calculate
ID: 1796126 • Letter: S
Question
s s6. () Caleulate the rate of heat conduction in watts out window is, calculate it for a window of area 3.0 and thickness 0.80 em if the temperatures at the outer and inner surfaces are 5.0 and 10°C respec tively an animal with 3.0-em thick fur. The surface area of the animal is 1.5 m*, its skin temperature is 35 C and the temperature of the surrounding air is 0'C. Heat losses due to conveetion and conduction can be 5.44. neglected, and the thermal conductivity of the fur can be assumed to be the same as that for air. Note that this is not much different than for a human in a warm room, as seen in Example 5.8 If 5.0 g of water evaporate from 150 g of soPc coffee, what will the final temperature of the coffee be? Assume the styrofoam eup is such a good insala- tor that all other forms of heat transfer can be ig- nored. Note that the "steam" above the coffce is really recondensed water vapor droplets. I) Caleulate the rate of heat conduetion through the walls of a house if they are 8.0 em thiek and have 5.45. (II1) What is the temperature differeneo across a twice the thermal conduetivity as glass wool. The total area of the walls is 120 m2 and the inside tem- perature is 20°C, while the outside temperature is 5°C. Note that this is only conduction through the double-paned window of area 3.0 m2 if each pane of glass is 1.0 cm thiek, as is the air gap between them, and the rate of heat conduction is 200 W. Hint: Ca culate the tempersture difference across each layer separately and add them together. You may assume that convection in the air gap is negligible. walls and does not include conduction through the OT 5.38. ) If a small ieeberg with a mass of 20 million kg 5.46. (1I1) (a) A glass coffee pot has a bottom with an moves south from the Arctie, how much beat is re- quired to melt the iceberg? Note that this amount of energy was released earlier at the origin of the ice- berg area of 400 em2, and the coffee in the pot has a tem- perature of 100C. If the bottom is 075 cm thick, how hot must the underside be to conduct heat into the pot at the rate of 500 w (b) How many grams of water boil away each second if convection plus boiling is the only method of energy transfer out of the pot? . 1) What is the rate of heat loss by radiation from a man completely clothed in white (head to foot) if his skin temperature is 34 and the surrounding tem perature is 10°C? His surface area is 1.4 m2 and the 5.47. (aII) Considerable temperature decreases can emissivity of the elothing is 0.2 oecur when cold air blows through an open door. If 20 of 0°C air enters a room, how much heat is 1) Whst is the rate of heat los by rnadiation from a black roof of arca 250 m2 if its temperature is 20*%c and that of the surroundings is 10°C? The emissivity 5.40. required to warm it to 20 C? of the roof is 0.95. 5.41. () Glowing embers in a fireplace have a tempera ture of 850 C, an emissivity of 0.98, and a surface Section 5.5 ealories of heat will a person lose by ings is 230 and 50% of the radiation from, the fire enters the room, what is the rate of heat transfer evaporating 1250 g of water and perspiration in a day? Caleulate how much perspiration a person must evaporate during physieal exercise to get rid of 100,000 cal of heat. into the room in kilowatts? 6.49. (I) through an 8.0-em-thick wall of area 10 m2 with that through 5.42. (II) Compare the rate of heal conduction 0.75-cm-thick window of area 2.0 m2, for the same 5.50. () How many calories of heat would have to be put temperature difference across each. The thermal conduetivity of the wall can be assumed to be twice that of glass wool. (II) It is usually cold next to a window on a winter day because heat conduction through the window is into the shoulder of a person receiving a heat treat- effect of eireulation and other factors can be ne- ration he will still evaporate about 600 g of water ment to raise the shoulder's temperature by 5°C The mass of the shoulder region is 5.0 kg, and the ted rapid enough to cool the air next to it. To see just 5.51. (n) On a day when a man shows no visible perspi- how large the rate of heat conduction through a glassExplanation / Answer
Thermal conductivity of glass wool = 0.04 W / m K
of wall, k =2 (0.04) = 0.08 W / m K
and Q/t = k A delta(T) / d
= (0.08) (120) (20 - 5) / (0.08)
= 1800 Watt Or 1.80 kW
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